The present invention relates to an information reproducing method using light emitted from a laser, and in particular, to a layer discriminating method indispensable to reproduce information on a multilayer optical disk using a blue-violet laser diode and a high-numerical-aperture objective and a method of detecting and correcting spherical aberration in the reproducing of information.
Picture or image recording apparatuses broadly used today generally utilize videotapes. Recently, a picture recording apparatus using an optical disk has been put to the market. The optical disk is quite suitable to access information thereon in a random fashion and has high usability. Quality of pictures on the optical disk hardly changes even after repetitious reproduction thereof and with the lapse of time. Moreover, the optical disk has only quite small volume. Therefore, it can be considered that the picture recording apparatuses will commonly use optical disks in the future. The optical disk is employed for various purposes in addition to the recording of information, for example, as an external storage of a computer and a music recording and reproducing device. Therefore, the optical disk expectedly becomes more important in the future.
In the telecasting, the satellite broadcast and the ground-wave broadcast will be digitized within a target period ranging from 2003 to 2005. In association therewith, it is considered that broadcast of mobile pictures finer in picture quality than the broadcast of today will be widely spread for family use and demands for digital recording of such highly-fine mobile pictures become stronger. To record the mobile pictures for about two hours with the picture quality kept unchanged, it is necessary to store a large amount of data of about 20 gigabytes (GB) 25 GB on a disk with a diameter of 12 centimeters (cm), the diameter being substantially equal to that of a compact disk or a DVD. That is, the recording capacity needs to be about five times that of the DVD today.
To increase the recording density, it is required that the laser light source has a shorter wavelength and the numerical aperture of an objective is increased to thereby reduce the diameter of spots for the information recording and reproducing operations. The recording capacity can be increased by using a multilayer recording technique for advantages of optical disks. At present, the laser light source of a DVD has a wavelength of about 660 nanometers (nm) and a numerical aperture of about 0.6, and the recording capacity is 4.7 GB on a single surface of one disk layer. As a laser light source having a shorter wavelength, a blue-violet laser diode (with a wavelength of 400 nm) is being put to practical uses. With this laser light source, a recording capacity of 50 GB is obtained on a single-sided, dual-layered disk when the numerical aperture is 0.85.
Such a single-sided optical disk having a recording capacity of 25 GB for each layer is described in, for example, pages 937 to 942 of Jpn. J. Appl. Phys. Vol. 39 (2000). Two groups of objectives are used to obtain a maximum numerical aperture of 0.85. When the numerical aperture is increased, there arises a problem of increase in aberration due to fluctuation in an optical system and errors in thickness and inclination of a disk substrate. To overcome the problem in the prior art example described above, the thickness of the substrate is reduced down to 0.1 millimeter (mm) to minimize coma aberration caused by the disk inclination. To compensate for spherical aberration caused by the error in the thickness of the substrate, the position of a collimator lens is changed.
Methods to detect, as an electric signal, spherical aberration caused by the error in the substrate thickness are described in pages 2292 to 2295 of Jpn. J. Appl. Phys. Vol. 40 (2001) and pages 1842 and 1843 of Jpn. J. Appl. Phys. Vol. 41 (2002). In the methods of the prior art, focus position fluctuation signals in a near-axis area and a far-axis area of a flux of reflected light from an optical disk are respectively detected to obtain a differential signal therebetween as a spherical aberration signal. By feeding the signal back to a liquid-crystal (LC) spherical aberration correcting element, the spherical aberration is successfully detected and corrected.
A rewritable dual-layered disk with a recording capacity of about 50 GB is described in pages 31 to 36 of Technical Report of IEICE, MR2001-93 Vol. 101 No. 564 (2002) and a write-once quadruple-layered disk with a recording capacity of about 100 GB is described in pages 33 to 35 of Proceedings of international symposium for three-dimensional memories (2002). As above, the recording capacity can be increased by using a multilayer recording disk for advantage of the optical disk. In these multilayer recording disks, to reduce crosstalk due to record signals of another layer down to a level to prevent adverse influence thereof, the gap between the recording layers ranges from about 20 micrometers (μm) to about 30 μm. Spherical aberration due to the gap is 0.2 rλrms or more and remarkably exceeds the Marechal's condition (wavefront aberration is at most 0.07 rλrms). Therefore, it is required to correct the spherical aberration in the recording and reproducing operations of a multilayer recording disk.
JP-A-2002-100061 describes a method of reproducing a multilayer recording disk. When a multilayer disk is reproduced according to the prior art example, spherical aberration is beforehand corrected for a predetermined quantity thereof, a recording layer is moved or an autofocus operation is conducted to obtain a reproduced signal for which spherical aberration is corrected.    [Non-patent article 1] Jpn. J. Appl. Phys. Vol. 39(2000)937–942    [Non-patent article 2] Jpn. J. Appl. Phys. Vol. 40(2001)2292–2295    [Non-patent article 3] Jpn. J. Appl. Phys. Vol. 41(2002)1842–1843    [Non-patent article 4] Technical report of IEICE, MR2001-93 Vol. 101 No.564(2002)31–36    [Patent article 1] JP-A-2002-100061